found this interesting reading:
"...An average car battery has an amp-hour rating of about 50 Ah. But does this mean it can put out 50A for an hour, or that it can put out 1A for 50 hours? The interesting thing about batteries is that the faster you try to pull out the electrons, the less it can provide. If you had a battery rated at 50 Ah, you could put out far more than 50 hours if you drew only 1A. For instance, if you ran all your interior lights (together, they might add up to 1A) on a 50 Ah battery, you could easily go 50 hours (about 2 days) or more. A good battery might last as long as a week.
However, if you hooked up some item that drew 50A, it would be pulling 50A x 12V = 600W. This would be like powering 10 normal indoor light bulbs, and it would kill your battery in just a few hours or less.
To be more precise, battery vendors designate the number of hours over which their amp-hour ratings applyusually 20 hours. By specifying "50 Ah @ 20 hours," they are saying that you get 50 Ah if you drain the battery over 20 hours. That works out to about 2.5A per hour (50 Ah / 20 hours = 2.5A). In other words, if you draw 2.5A, you should get exactly 20 hours of life out of a fully charged 50-Ah battery. If you draw only 1A, you should get more than 20 hours, but it's hard to say how much moreperhaps 40+ hours. If you draw 5A, you will get less than 20 hours, and probably less than 10 hours because you're draining it above its specification.
So, if you know a battery has a specific Ah rating, just divide it by 20 to get the amperage at which you could run your devices for 20 hours. By keeping your device amperage under the value you get, you should be able to get your 20 hours or more of power.
When you get to the end of the 20 hours, the battery is not completely drained. However, it will have dropped below 11 volts, and it is for all intents and purposes "discharged," in the sense that the voltage is too low to start your car or power your device. The battery most likely also will be damaged and will have a shorter life span, as conventional car-starting batteries are not designed for deep discharge...".
assume i have a 58 ah @ 20 hours battery, i could draw approx. 3 a (58 / 20) for twenty hours before battery is discharged. or running my ion (3 a x 12 v = 36 watts) for 20 hours while putting fuel (should be enough even to have a coffe, go shopping).
i get one!